We present a set of 148 independent N -body simulations of globular clusters (GCs) computed using the code CMC (Cluster Monte Carlo). At an age of ∼ 10−13 Gyr, the resulting models cover nearly the full range of cluster properties exhibited by the Milky Way GCs, including total mass, core and half-light radii, metallicity, and galactocentric distance. We use our models to investigate the role that stellarmass black holes play in the process of core collapse. Furthermore, we study how dynamical interactions affect the formation and evolution of several important types of sources in GCs, including low-mass X-ray binaries, millisecond pulsars, blue stragglers, cataclysmic variables, Type Ia supernovae, calciumrich transients, and merging compact binaries. While our focus here is on old, low-metallicity GCs, our CMC simulations follow the evolution of clusters over a Hubble time, and they include a wide range of metallicities (up to solar), so that our results can also be used to study younger and higher-metallicity star clusters.2. METHODS 2.1. Summary of CMC

Black holes formed in dense star clusters, where dynamical interactions are frequent, may have fundamentally different properties than those formed through isolated stellar evolution. Theoretical models for single-star evolution predict a gap in the black hole mass spectrum from roughly 40-120 M e caused by (pulsational) pairinstability supernovae. Motivated by the recent LIGO/Virgo event GW190521, we investigate whether black holes with masses within or in excess of this "upper-mass gap" can be formed dynamically in young star clusters through strong interactions of massive stars in binaries. We perform a set of N-body simulations using the CMC clusterdynamics code to study the effects of the high-mass binary fraction on the formation and collision histories of the most massive stars and their remnants. We find that typical young star clusters with low metallicities and high binary fractions in massive stars can form several black holes in the upper-mass gap and often form at least one intermediate-mass black hole. These results provide strong evidence that dynamical interactions in young star clusters naturally lead to the formation of more massive black hole remnants.

Recent numerical simulations of globular clusters (GCs) have shown that stellar-mass black holes (BHs) play a fundamental role in driving cluster evolution and shaping their present-day structure. Rapidly mass-segregating to the center of GCs, BHs act as a dynamical energy source via repeated superelastic scattering, delaying the onset of core collapse and limiting mass segregation for visible stars. While recent discoveries of BH candidates in Galactic and extragalactic GCs have further piqued interest in BH-mediated cluster dynamics, numerical models show that even if significant BH populations remain in today’s GCs, they are not typically in directly detectable configurations. We demonstrated in Weatherford et al. that an anticorrelation between a suitable measure of mass segregation (Δ) in observable stellar populations and the number of retained BHs in GC models can be applied to indirectly probe BH populations in real GCs. Here we estimate the number and total mass of BHs in 50 Milky Way GCs from the Advanced Camera for Surveys GC Survey. For each GC, Δ is measured between observed main-sequence populations and fed into correlations between Δ and BH retention found in our CMC Cluster Catalog’s models. We demonstrate that the range in measured Δ from our models matches that for observed GCs to a remarkable degree. Our results constitute the largest sample of GCs for which BH populations have been predicted to date using a self-consistent and robust statistical approach. We identify NGC 2808, 5927, 5986, 6101, and 6205 to retain especially large BH populations, each with a total BH mass exceeding 103 .

Numerical and observational evidence suggests that massive white dwarfs dominate the innermost regions of corecollapsed globular clusters by both number and total mass. Using NGC 6397 as a test case, we constrain the features of white dwarf populations in core-collapsed clusters, both at present day and throughout their lifetimes. The dynamics of these white dwarf subsystems have a number of astrophysical implications. We demonstrate that the collapse of globular cluster cores is ultimately halted by the dynamical burning of white dwarf binaries. We predict that core-collapsed clusters in the local universe yield a white dwarf merger rate of -- 10 Gpc yr 3 1 ( ) , roughly 0.1%-1% of the observed Type Ia supernova rate. We show that prior to merger, inspiraling white dwarf binaries will be observable as gravitational-wave sources at millihertz and decihertz frequencies. Over 90% of these mergers have a total mass greater than the Chandrasekhar limit. We argue that the merger/collision remnants, if not destroyed completely in an explosive transient, may be observed in core-collapsed clusters either as young neutron stars/pulsars/magnetars (in the event of accretion-induced collapse) or as young massive white dwarfs offset from the standard white dwarf cooling sequence. Finally, we show that collisions between white dwarfs and main-sequence stars, which may be detectable as bright transients, occur at a rate of -- 100 Gpc yr 3 1 ( ) in the local universe. We find that these collisions lead to depletion of blue straggler stars and main-sequence star binaries in the centers of core-collapsed clusters.

We explore the possibility that GW190412, a binary black hole merger with a non-equal-mass ratio and significantly spinning primary, was formed through repeated black hole mergers in a dense super star cluster. Using a combination of semianalytic prescriptions for the remnant spin and recoil kick of black hole mergers, we show that the mass ratio and spin of GW190412 are consistent with a binary black hole whose primary component has undergone two successive mergers from a population of black holes in a high-metallicity environment. We then explore the production of GW190412-like analogs in the CMC Cluster Catalog, a grid of 148 N-body star cluster models, as well as a new model, behemoth, with nearly 107 particles and initial conditions taken from a cosmological MHD simulation of galaxy formation. We show that, if the spins of black holes born from stars are small, the production of binaries with GW190412-like masses and spins is dominated by massive super star clusters with high metallicities and large central escape speeds. While many are observed in the local universe, our results suggest that a careful treatment of these massive clusters, many of which may have been disrupted before the present day, is necessary to characterize the production of unique gravitational-wave events produced through dynamics.